Thermal Decomposition and Combustion of Microwave Pre-Treated Biomass Pellets

Abstract

The objective of the study was to investigate a more effective use of commercially available biomass pellets (wheat straw, wood, peat) using microwave pretreatment to improve heat production. Pellets were pretreated using the originally designed microwave torrefaction device. The effects of microwave (mw) pretreatment were quantified, providing measurements of the weight loss and elemental composition of pellets and estimating the effect of mw pretreatment on their porosity, surface area and calorific values at pretreatment temperatures of T = 448–553 K. Obtained results show that the highest structural variations and elemental composition during mw pretreatment were obtained for wheat straw pellets, with an increase in reactivity, a decreasing in the duration of the thermal decomposition by about 40% and an increase in the yield of combustible volatiles. Increased reactivity of pretreated pellets enhanced the ignition and burnout of volatiles, decreasing the duration of the burnout of pretreated wheat straw, wood and peat pellets by 40%, 24% and 9%, respectively, and increasing the peak and average values of the flame temperature, heat output, and produced heat energy by 40–50%, with a correlating increase of combustion efficiency and the mass fraction of carbon-neutral CO2 emission. Thus, the applicability of microwave pretreatment for the control and improvement of heat production was confirmed.

Activated combustion of biomass blends by microwave pre-treatment of straw

“…1c). Measurements of the carbon content in pretreated pellets show carbonization of wheat straw pellets during MW pre-treatment [11] increasing HHV of pre-treated pellets (Fig. 1d).…”

“…Secondly, an experimental device with a heat power up to 5 kW consisting of a gasifier and a combustor [11] was used to study the combustion characteristics of blends of commercially available, untreated wood pellets with 3 types of wheat straw pellets: untreated and pre-treated at Tmw = 473 K and Tmw = 548 K. The gasifier was filled with a constant volume and total mass 340-480 g of biomass blends. The thermal decomposition of blends was initiated using an additional heat supply by propane flame flow and was supported using the primary air supply below the layer of blend with average rate 0.8 g•s -1 , thus enhancing the yield of volatiles (CO, H2, CxHy, CO2), The effect of MW pre-treatment was studied by varying the mass fraction of wheat straw in the blends from 15% to 60%.…”

Improvement of Biomass Gasification/Combustion Characteristics by Microwave Pretreatment of Biomass Pellets

“…Besides, analysis of the produced gas was used to estimate combustion efficiency and composition of emissions. The methodology of measurements of the combustion characteristics is described in [17].…”

“…The porous structure of the pretreated wheat straw pellets was evaluated from N 2 sorption/desorption isotherms determined in a sorptometer Quntachrome NOVA 4200e. Degassing was performed at room temperature for 120 h. The specific surface area was assessed using Quantachrome software and is based on the theories by Brunauer-Emmet-Teller [17].…”

“…Comparison of MW-pretreated biomass fuels (wood, wheat straw, and peat pellets) suggests that the increased surface area, porosity, and reactivity enhances their thermal decomposition during gasification by increasing the yield of volatiles and decreasing the duration of thermal decomposition [15]. Besides, the enhanced carbonization of lignocellulosic pellets during their MW pretreatment also promotes an increase of HHV up to 12.7 % for wheat straw pellets, 20.8 % for wood pellets, and 26.8 % for peat pellets [17].…”